the four that split on our bike all split right down the middle along the spokes. We rode the Dalton Highway in Alaska, but other than that, all our riding had been on relatively good roads. The weight on the bike wasn't all that much in that we had a small child (about 75 pounds) on the back and most of the weight of the gear was in a BOB.
I believe our friends' rims split the same way - right down the center. They are running a lot more weight as they do not have trailers at all. They have split both the front and rear rims. They have ridden quite a few miles on rough, dirt roads, but only have about 4500 miles total on their bikes and have gone through 6 rims in those 4500 miles!